Nanatsu No Hikan

One day, a young man from a high-classed background named Ippei is called to the office of his uncle, Shujiro. Shujiro is the current head of his family who is concerned about seven mansions left behind by the last clan head (Ippei's grandfather, Shuhei). The seven mansions were strangely built in a Big Dipper formation on a remote island and have never been fully explored by the family members of Ippei's generation. Shujiro wants to create a resort where the mansions are located yet feels that there is a marvelous treasure or secret left behind by their ancestors. Too aged and busy to explore the buildings himself, Shujiro asks Ippei to unlock the secrets for him. He gives the eager youth a key to the first mansion and a riddle left behind by Shuhei.